Apparatus for measuring bending on a drill bit operating in a well

ABSTRACT

An apparatus for measuring the bending on a drill bit operating down hole in a well may comprise at least three pockets circumferentially spaced equidistantly around the drill collar of the drill string to which the drill bit is attached. Four strain gauges are equidistantly circumferentially spaced around each of the pockets so as to form first and second sets of strain gauges. The strain gauges in the first set are connected into one Wheatstone bridge while the gauges in the second set are connected in a second bridge. Each of the strain gauges that are oriented similarly within each of the pockets are connected in series within a single leg of a bridge so that the output voltage of the bridge is unaffected by bending in the drill string. The output of first bridge is used to determine the bending on the drill bit. Further, an apparatus for measuring the bending, weight, and torque on a drill bit operating down hole in a well may comprise at least three pockets circumferentially spaced equidistantly around the drill collar of the drill string to which the drill bit is attached with twelve strain gauges that are equidistantly circumferentially spaced around each of the pockets so as to form first and second sets of strain gauges. The arrangement of the twelve strain gauges allow for a bending on bit measurement that cancels out the affects of the weight on bit and torque on bit.

This application is a divisional of U.S. application Ser. No.12/512,740, filed Jul. 30, 2009, now issued as U.S. Pat. No. 8,397,562,the entire disclosure of which is incorporated by reference into thisapplication.

FIELD

The current invention is directed to an apparatus for measuring bendingon a drill bit. More specifically, the current invention is directed tothe measurement of the bending on a drill bit operating down hole in awell, such as an oil well.

BACKGROUND

In underground drilling, such as gas, oil, or geothermal drilling, abore is drilled through a formation deep in the earth. Such bores areformed by connecting a drill bit to sections of pipe, referred to as“drill pipe,” so as to form an assembly commonly referred to as a “drillstring” that is suspended from a rig at the surface and that extendsdown to the bottom of the bore. The drill bit is rotated so that itadvances into the earth, thereby forming the bore. In rotary drilling,the drill bit is rotated by rotating the drill string at the surface. Indirectional drilling, the drill bit is rotated by a down hole mud motorcoupled to the drill bit; the remainder of the drill string is notrotated during drilling. In a steerable drill string, the mud motor isbent at a slight angle to the centerline of the drill bit so as tocreate a side force that directs the path of the drill bit away from astraight line. In any event, in order to lubricate the drill bit andflush cuttings from its path, piston operated pumps on the surface pumpa high pressure fluid, referred to as “drilling mud,” through aninternal passage in the drill string and out through the drill bit. Thedrilling mud then flows to the surface through the annular passageformed between the drill string and the surface of the bore.

Depending on the drilling operation, the pressure of the drilling mudflowing through the drill string will typically be between 0 and 25,000psi. In addition, there is a large pressure drop at the drill bit sothat the pressure of the drilling mud flowing outside the drill stringis considerably less than that flowing inside the drill string. Thus,the components within the drill string are subject to large pressureforces. In addition, the components of the drill string are alsosubjected to wear and abrasion from drilling mud, as well as thevibration of the drill string.

Throughout the drilling operation, a drill bit may be subject to variousloads that act on the drill string. The fundamental loads acting on thedrill string are: axial tension, torsion, bending, pressure andtemperature. All of these loads result in strain being applied to thedrill string. These loads may be static or dynamic and fluctuate duringthe drilling process. The axial tension loads are due to applying adrilling weight to the drill bit. This is normally referred to“weight-on-bit” or WOB. The actual amount of weight-on-bit depends onthe entire weight of the drillstring and the amount tensile load appliedat the rig. This is typically referred to as “hook load”. Secondaryloads that effect the weight applied to the bit are hydrostatic loadsand friction loads. The hydrostatic loads depend on the depth at the bitand the density of the drilling fluid. There are also friction loadsalong the length of the drillstring due to contact between thedrillstring and the borehole. Torsion loads are applied to the drillstring to provide adequate cutting torque at the bit. Bit torque may isapplied by two means; by a motor at the rig that rotates the drillstring that in turn rotates the bit, and/or by a down hole motor thatrotates the a drive shaft just above the bit. Drill string bending comesfrom the well path curvature, sagging of the drill string due togravitational forces and buckling. Typically, as the well is drilldeeper the temperature increases. However, there may be temperaturevariations within different formations. The annulus temperature and thetemperature within the fluid within the bore of the drill string may beslightly different. Bore temperatures tend to be slightly cooler thanthe annulus temperatures.

The loads applied to the drill string strain the drill string material.Strain gauges can be used to measure these loads. The strain gauges arepositioned on a drill collar such that the are subjected to certainstrains. The gauges are connected in a bridge arrangement such that themeasure the desired load while eliminating the effects of other loads.For example, the gauges can be mounted on a drill collar and connectedto one another such that the bridge measures only the bending load,subtracting out tensile and torque loads. Pressure and temperaturestains can not be cancelled out. Therefore, these loads and measured anda correction factor is applied to the measurement. The measurements maybe recorded down hole and/or transmitted to the surface by mud pulse orwired pipe.

U.S. Pat. No. 6,547,016, entitled “Apparatus for Measuring Weight andTorque on a Drill Bit Operating in a Well,” hereby incorporated byreference in its entirety, provides methods for calculating weight onbit and torque on bit in such a way that the effects of bending do noteffect the calculations, but does not include any manner for measuringthe effects of bending.

SUMMARY

Methods of measuring bending loads are lacking in conventional weight onbit and torque on bit measurement systems. Disclosed herein are methodsand an apparatus for measuring the bending on a drill bit. The benefitsof incorporating strain gauges that measure bending loads in the mannerdisclosed have not been realized. Such apparatus comprises first,second, and third pockets formed in a drill pipe, where the pockets arecircumferentially spaced approximately equidistantly around the pipe.Each of the pockets forms at least one wall, and a set of strain sensorsis affixed to the wall in each pocket. Circuitry connects each of thestrain sensors in each set, forming a bridge with a first, second,third, and fourth legs. The bridge is arranged in a manner to enable theapparatus to sense the bending of a drill bit.

Further, an apparatus may comprise a similar arrangement of sensors thatcan measure weight and/or torque applied to a drill bit. In such anapparatus, a set of sensors for each measurement type may be arrangedand connected via circuitry to form a bridge. The bridge may be arrangedto enable each set of sensors to separately measure the bending, weight,and torque on a drill bit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view, partially schematic, of a drilling rig in which thedrill string incorporates an apparatus for measuring bending on thedrill bit according to the present invention.

FIG. 2 is a view of a portion of a drill collar.

FIG. 3 is a longitudinal cross-section through the drill collar portionof the drill string shown in FIG. 1.

FIG. 4 is a transverse cross-section taken along line III-III in FIG. 3.

FIG. 5A is an elevation view taken along line IV-IV in FIG. 3 lookinginto the pocket, with the plug removed, showing the orientation of thebending on bit strain gauges.

FIG. 5B is an isometric view of the pocket shown in FIG. 5A.

FIG. 6 is schematic diagram of the system for measuring the bending onthe drill bit according to the current invention.

FIG. 7 is a view taken along line VI-VI in FIG. 5A showing a portion ofthe pocket side wall to which the bending on bit strain gauges areaffixed.

FIG. 8A is an elevation view taken along line IV-IV in FIG. 3 lookinginto the pocket, with the plug removed, showing the orientation of theweight on bit, torque on bit, and bending on bit strain gauges.

FIG. 8B is an isometric view of the pocket shown in FIG. 8A.

FIGS. 9( a), (c), and (d) show exaggerated views of the distortion of apocket under compression, tension, and torsion, respectively. FIG. 9( b)depicts another example of the strain pattern in the hole for tensionand torque.

FIG. 10 is a schematic diagram of the system for measuring the tension,torque, and bending on the drill bit according to the current invention.

FIG. 11 is a view similar to FIG. 4 showing an alternate embodiment ofthe disclosed techniques.

FIG. 12 is a detailed view of one of the pockets shown in FIG. 4.

FIG. 13 is an alternate embodiment of the pocket shown in FIG. 12.

DETAILED DESCRIPTION

Disclosed herein are techniques for measuring bending on bit (BOB). Abending load is the bending of the longitudinal axis of the drillstring. Information concerning the bending on the drill bit can provideuseful information for the drilling operator. For example, a drill pipesubjected to a bending strain as it rotates experiences a cyclicallyvarying bending stress, which can lead to a deviation of the drill bitfrom its intended course. Thus, information concerning the bending onthe drill bit can alert an operator to take appropriate correctiveaction to return the drill bit to its intended path. A sufficientlylarge bending load can cause fatigue-damage on each revolution. Ifknown, the operator can take corrective measures to decrease the amountof bending, which may include replacing deformed sections of the drillstring. In this manner, information concerning the bending on bit canassure that the target formation is drilled within tolerance limits,helping to avoid wasted drilling time. Further, the operator can usebending on bit information for better directional control of the drillstring.

A drilling system that can employ the disclosed techniques of measuringbending on bit is shown in FIG. 1. The system comprises a derrick 5 thatsupports a drill string 4. A drill bit 8 is coupled to the distal end ofa drill collar section 6 of the drill string 4. A drill bit may be anysuitable drill bit using in a drilling operation, including conventionaldrill bits, coring bits, and reamers. The drill bit 8 forms a bore 2 inthe earthen formation 3. The weight on the drill bit 8 is controlled byvarying the hook load on the derrick 5. A prime mover (not shown) drivesgearing 7 that rotates the drill string 4 so as to control the torque onthe drill bit 8.

As is conventional, a pump 10 pumps drilling mud 14 downward through aninternal passage 18, shown in FIG. 2, in the drill string 4. Afterexiting at the drill bit 8, the returning drilling mud 16 flows upwardto the surface through an annular passage formed between the drillstring 4 and the bore 2. As is also conventional, a data acquisitionsystem 12 at the surface senses pressure pulsations in the drilling mud14 created by a mud pulser 5 that contain encoded information concerningthe drilling operation. A bending moment may be imposed on the drill bitby the reaction forces on the bit and the submerged weight of the drillstring in the drilling fluid (e.g., mud). The angle in direction of thedrill string can cause bending, particular at the joint in the bore holewhere the direction changes. The more weight on in the drill string(e.g., the number of interconnected pipes suspended from the top driveassembly) may result in more stress at that joint.

FIG. 2 shows three of the primary loads of interest acting on the drillstring. The tensile load is a force, or forces, 19 a, 19 b, that attemptto stretch or compress the drill string along the longitudinal axial ofthe drill collar. The torsion load, 20 a, 20 b, attempts to twist thedrill string about the longitudinal axis. The bending load, 21 a, 21 b,is bending of the longitudinal axis. The bending load on the drillstring may result from the curvature of the hole through which the drillstring is boring. For example, if the desired bore-hole to form in afoundation is not directly downward into the earth, but rather changesdirection or is to be bored at an angle (as shown in FIG. 1), the drillstring bends to accommodate the directional changes. The weight on bitcan cause buckling or bending stress at various points along the drillstring. The amount of “weight on bit” may vary by adjusting the weightapplied to the drill bit when suspending, from a top drive assembly, asuccession of drill collars and drill pipes that are screwed together toform the drill string. When rotating the drill string, the bendingstress may change, which may modify the tensile stress values. Asdescribed in more detail below, strain gauges may be mounted in acircular pocket 17 in the drill collar 6 in suitable positions tomeasure the tensile, torsion, and bending loads acting on the drillstring.

The drill collar 6 is shown in detail in FIGS. 3 and 4. As isconventional, the drill collar 6 is formed from a section of drill pipehaving threaded connections at each end (not shown) that allow it to becoupled into the drill string. For example, one end of the drill collaris coupled to the drill bit 8 from FIG. 1 while the other end is coupledto an uphole section of the drill string. According to the disclosedtechniques, three pockets 37, identified as P1, P2 and P2 in FIG. 4, arecircumferentially spaced equidistantly around the circumference of thedrill collar 6. One example pocket, P2, is visible in FIG. 3 becauseFIG. 3 depicts a cross-section view of the drill collar portion of thedrill string. Preferably, the pockets, P1, P2, and P3 are located on acommon plane oriented perpendicularly to the centerline E of the drillcollar 6. Each pocket, P1, P2, P3, extends radially inward from thesurface of the drill collar 6 toward the centerline E so as to form acylindrical side wall 38 and a bottom wall 35 (see FIG. 5B). Each pocket37 is closed by a cap 36, which is secured to the drill collar 6 via asnap ring (not shown) and incorporates O-rings (not shown) that seal thepocket from the drilling mud 16.

As shown in FIGS. 5A and 5B, a first transversely extending passage 24connects pockets P1 and P2, and a second transversely extending passage22 connects pockets P2 and P3. As shown in FIG. 5A, an axially extendingpassage 34 connects pocket P2 to a recess 26 formed in the drill collar6. A circuit board 30 and microprocessor 32 are housed within the recess26, which is sealed with a cap 28. The passages 22, 24 and 34 permitelectrical conductors to extend between the pockets P1, P2 and P2 andbetween the pocket P2 and the recess 26 so as to complete the circuitrydescribed in detail below.

The drill bit located at the distal end of the drill string can berotated by rotating the drill string at the surface. Thus, the drillcollar 6 and pockets P1, P2, P3 may rotate. If the drill string anddrill collar are rotating, the axial tension or compression will varywith time. On the inside of the bend, the gauges will be in compressionand on the outside in tension. The measured bending will vary,approximately sinusoidally, as the pockets P1, P2, P3 rotate The bendingmoment will be equal to one-half of the difference between the maximumand minimum readings over a time that covers several rotation periods,i.e.,

${BOB} = {\frac{\left( {M_{\max} - M_{\min}} \right)}{2}.}$The results for all of the pockets are averaged for best results. Thismethod of measurement can eliminate the contributions to the bendingmeasurement that result from the weight on the drill bit or pressure, asthe WOB and pressure measurements will be equal for all pockets and notdependent upon the drill collar's orientation. The rotating method canbe used with one or more WOB bridges. To determine whether or not thedrill string or drill collar is rotating, in the absence of a rotationsensor, a limit on the variation of the individual readings can be used.For example, the following can define a non-rotating (or non-bending)condition:

$\frac{M_{\max} - M_{\min}}{M_{\max} + M_{\min}} < L$

where M is the WOB measurement of a given bridge

-   -   L is a limit (e.g., 1-3%).        If all three bridges produce results that meet this criterion,        then it may be assumed that the drill collar is rotating and the        method can be used. As shown in FIGS. 5A and 5B, conventional        strain gauges 39, such as foil or semiconductor type gauges, are        affixed to the side wall 38 of each of the pockets 36. The        details of the arrangement of the bending on bit (BOB) strain        gauges 39 are shown in FIG. 4 for pocket P2 but it should be        understood that the BOB strain gauges are arranged identically        in each of the pockets. As shown in FIG. 4A, four bending on bit        (BOB) strain gauges 39 are equidistantly spaced around the        circumference of the pocket side wall 38. With reference to the        angle of orientation shown for each of the strain gauges, the        four BOB gauges 39 are shown spaced around the circumference of        the pocket side wall 38 at angles 0, 90, 180, and 270        orientation (i.e., BOB-BOB-P2 ₀, BOB-P2 ₉₀, BOB-P2 ₁₈₀, and        BOB-P2 ₂₇₀).

As shown in FIG. 5B, the BOB gauges are positioned longitudinally in thedrill collar on the same plane. The strain gauges in each pocket P areelectrically connected so as to form three sets of strain gauges (i.e.,a set in each of the three pockets) each set comprised of four gauges(i.e., 4 gauges in each pocket). The strain gauges 39 in the first setof strain gauges in pocket P2 are identified as BOB-P2 ₀, BOB-P2 ₉₀,BOB-P2 ₁₈₀, and BOB-P2 ₂₇₀ and, together with similarly oriented straingauges in the other two pockets, are used to determine the bending onthe drill bit 8. Strain gauges BOB-P2 ₀ and BOB-P2 ₁₈₀ are disposed onopposite sides of the pocket side wall 38 and are located along a line Athat is parallel with the center line E of the drill collar 6 so thatBOB-P2 ₀ is located at the 0° circumferential orientation and BOB-P2 ₁₈₀is located at the 180° orientation, with 0° being top dead center of thepocket P2. Strain gauges BOB-P2 ₉₀ and BOB-P2 ₂₇₀ are also disposed onopposite sides of the pocket side wall 38 and located along a line Cthat is perpendicular to line A, and therefore to the center line E ofthe drill collar 6, so that BOB-P2 ₉₀ is located at the 90°circumferential orientation and BOB-P2 ₂₇₀ is located at the 270°orientation.

As shown in FIG. 6, the BOB measurement utilizes four gauges in each ofthe three collar pockets, P1, P2, P3. The four BOB strain gauges in thefirst set of strain gauges from each of the three pockets are formedinto a first Wheatstone bridge 90 comprised of twelve BOB strain gaugesarranged in four legs L₁, L₂, L₃, and L₄, with leg L₁ being opposite toleg L₂ and leg L₃ being opposite to leg L₄. Each leg, L₁, L₂, L₃, andL₄, uses three strain gauges, one from a similar position in each of thepockets. As shown, the BOB strain gauges at the 0° orientation in eachof the three pockets are connected in series along leg L₁, the BOBstrain gauges at the 180° orientation in each of the three pockets areconnected in series along leg L₃, the BOB strain gauges at the 90°orientation in each of the three pockets are connected in series alongleg L₂, and the BOB strain gauges at the 270° orientation in each of thethree pockets are connected in series along leg L₄. The junction formedby legs L₁ and L₃ forms a first input terminal I₁, while the junctionformed by legs L₂ and L₄ forms a second input terminal I₂. The junctionformed by legs L₂ and L₃ forms a first output terminal O₁, while thejunction formed by legs L₄ and L₁ forms a second output terminal O₂.

It may be desirable to measure the tensile load and torsion load on thedrill bit in addition to bending. Thus, WOB and TOB strain gauges may beaffixed to the pocket side wall, in addition to the BOB strain gauges.For example, FIG. 7 shows an example portion of a pocket side wall andthe arrangement of a BOB strain gauge, a WOB strain gauge, and a TOBstrain gauge. The weight on bit (WOB) gauges are positioned in each ofthe same orientations (i.e., 0, 90, 180, and 270) as the BOB gauges, theBOB and WOB strain gauges are shown at the same radial location on theside wall 38. The WOB gauges may be positioned above or below the BOBgauge in the pocket at each of the same orientations, 0, 90, 180, and270, as long as the WOB strain gauges are positioned longitudinallyalong the same plane, and the BOB strain gauges are positionedlongitudinally along the same plane. In FIG. 7, they are positionedbelow the BOB gauges.

Each strain gauge 39 is oriented so that its sensitive axis is orientedin the circumferential direction with respect to the cylindrical sidewall 38. The gauges are mounted in the circular pockets in positionssuch that the measurements of the individual tensile, torsion, andbending loads can be made. FIGS. 8A and 8B depict an example embodimentof a drill collar with all three types of strain gauges: eight strain(WOB) gauges, 4 torque on bit (TOB) gauges and 4 bending on bit (BOB)gauges 39, equidistantly spaced around the circumference of the pocketside wall 38. With reference to the angle of orientation shown for eachof the strain gauges, the four BOB gauges 39 are shown spaced around thecircumference of the pocket side wall 38 at angles 0, 90, 180, and 270orientation. At each location of a BOB gauge, in each of the sameorientations, 0, 90, 180, and 270, there may be a weight on bit (WOB)gauge. The WOB gauges may be positioned above or below the BOB gauge inthe pocket. In FIG. 8A, they are positioned below the BOB gauges asshown by the isometric view of the pocket in FIG. 8B, hence they are notvisible in the cross-section of the drill collar. The isometric view inFIG. 8B more clearly depicts both the BOB and the WOB gauges.

As shown in FIG. 8B, the TOB and WOB gauges are positionedlongitudinally in the drill collar on the same plane, and that plane islower than the plane on which the BOB gauges are positioned. The WOB,BOB, and TOB gauges can be positioned in similar orientations withrespect to their sensitive axes. The WOB gauges are positioned in thesame orientation as the BOB gauges, in the 0, 90, 180. and 270orientations, but each WOB gauge is positioned lower (i.e., into thepage) in the pocket than the BOB gauges, but on the same plane as theTOB gauges (see FIG. 5). Because the WOB gauges are positioned on adifferent plane, directly below each of the BOB gauges, they are notvisible in FIG. 6A. However, as shown in the isometric view of pocket P2in FIG. 6B, the WOB gauges are positioned on the same plane in thepocket as the TOB gauges, on a plane lower than the plane on which theBOB gauges are positioned. In another example embodiment, the BOB gaugescan be positioned in the same orientation as the WOB gauges, in the 0,90, 180. and 270 orientations, but the BOB gauges can be on the sameplane as the TOB gauges and the WOB gauges may be on a plane above orbelow the TOB gauges. It is contemplated that any of the three sets ofstrain gauges could be positioned on a longitudinal plane that is adifferent plane from another set.

The strain gauges in each pocket P are electrically connected so as toform three sets of strain gauges, each set comprised of four gauges. Thestrain gauges 39 in the first set of strain gauges in pocket P2 areidentified as BOB-P2 ₀, BOB-P2 ₉₀, BOB-P2 ₁₈₀, and BOB-P2 ₂₇₀ and,together with similarly oriented strain gauges in the other two pockets,are used to determine the bending on the drill bit 8. Strain gaugesBOB-P2 ₀ and BOB-P2 ₁₈₀ are disposed on opposite sides of the pocketside wall 38 and are located along a line A that is parallel with thecenter line E of the drill collar 6 so that BOB-P2 ₀ is located at the0° circumferential orientation and BOB-P2 ₁₈₀ is located at the 180°orientation, with 0° being top dead center of the pocket P2. Straingauges BOB-P2 ₉₀ and BOB-P2 ₂₇₀ are also disposed on opposite sides ofthe pocket side wall 38 and located along a line C that is perpendicularto line A, and therefore to the center line E of the drill collar 6, sothat BOB-P2 ₉₀ is located at the 90° circumferential orientation andWOB-P2 ₂₇₀ is located at the 270° orientation.

The second set of strain gauges 39 in pocket P2 are identified as TOB-P2₄₅, TOB-P2 ₁₃₅, TOB-P2 ₂₂₅, and TOB-P2 ₃₁₅ and, together with similarlyoriented strain gauges in the other two pockets, are used to determinethe torque on the drill bit 8. Strain gauges TOB-P2 ₄₅ and TOB-P2 ₂₂₅are disposed on opposite sides of the pocket side wall 38 and locatedalong a line B that is oriented 450 to the center line E of the drillcollar 6 so that TOB-P2 ₄₅ is located at the 45° circumferentialorientation and TOB-P2 ₂₂₅ is located at the 225° orientation. Straingauges TOB-P2 ₁₃₅ and TOB-P2 ₃₁₅ are also disposed on opposite sides ofthe pocket side wall 38 and are located along a line D that isperpendicular to line B, and therefore is also oriented at 45° to thecenter line E of the drill collar 6, so that TOB-P2 ₁₃₅ is located atthe 135° circumferential orientation and TOB-P2 ₃₁₅ is located at the315° orientation.

The third set of strain gauges 39 in pocket P2 can be weight on bitgauges, shown in FIG. 5B but not visible in FIG. 8A due to each of 4WOB, WOB-P2 ₀, WOB-P2 ₉₀, WOB-P2 ₁₈₀, and WOB-P2 ₂₇₀ gauges beingpositioned directly beneath each of the 4 BOB gauges shown. Togetherwith similarly oriented strain gauges in the other two pockets, are usedto determine the weight on the drill bit 8. Similar to strain gaugesBOB-P2 ₀ and BOB-P2 ₁₈₀, strain gauges WOB-P2 ₀ and WOB-P2 ₁₈₀ can bedisposed on opposite sides of the pocket side wall 38 along a line Athat is parallel with the center line E of the drill collar 6 so thatWOB-P2 ₀ is located at the 0° circumferential orientation and WOB-P2 ₁₈₀is located at the 180° orientation, with 0° being top dead center of thepocket P2. Also, similar to strain gauges BOB-P2 ₉₀ and BOB-P2 ₂₇₀,strain gauges WOB-P2 ₉₀ and WOB-P2 ₂₇₀ can also disposed on oppositesides of the pocket side wall 38 and located along a line C that isperpendicular to line A, and therefore to the center line E of the drillcollar 6, so that WOB-P2 ₉₀ is located at the 90° circumferentialorientation and WOB-P2 ₂₇₀ is located at the 270° orientation. The WOBgauges may be positioned below the BOB gauges, as shown in FIG. 8B, orthe WOB gauges may be located above the BOB gauges.

As previously discussed, each individual set of strain gauges in pocketsP1, P2, and P3 is arranged identically.

With respect to FIGS. 9A-9D, it is assumed that, similar to theembodiment in FIG. 4, the WOB and TOB gauges are positioned on the sameplane laterally in the pocket, and each BOB strain gauge is positionedhigher than each WOB strain gauge in the same orientation (0, 90, 180,and 280). However, it is noted that the BOB gauges may be placed eitherabove, on top, or below the WOB gauges. The arrangement is representedas W/BOB, indicating that both a WOB and a BOB strain gauge are locatedat the particular orientation in some radial order, with one type ofgauge located above the other type. In FIG. 9( a), when the portion ofthe drill collar 6 in the vicinity of a pocket P is subjected to pureaxial compression, the strain gauges WOB₀ and WOB₁₈₀ are placed intension, while strain gauges WOB₉₀ and WOB₂₇₀ are placed in compression.

In FIG. 9( b), when the portion of the drill collar 6 in the vicinity ofa pocket P is subjected to pure axial tension, the strain gauges W/BOB₀and W/BOB₁₈₀ are placed in compression, while strain gauges W/BOB₉₀ andW/BOB₂₇₀ are placed in tension. The WOB and BOB strains at each of thetorque gauge locations for the same particular gauge are the same.However, the strain is not zero. The TOB bridge arrangement is designedto cancel out these strains. Strains developed from torque loads aredissimilar and occur in different directions at the torque gaugelocations. The TOB bridge is arranged to measure these strains. Theaxial tension can result in a bending stress and/or strain on the drillbit. The BOB₀, BOB₁₈₀, BOB₉₀, and BOB₂₇₀ strain gauges measure thebending that results from the bending. The WOB₀, WOB₁₈₀, WOB₉₀, andWOB₂₇₀ strain gauges measure the strain that results from the axialtension.

FIG. 9( c) depicts another example of the strain pattern in the hole fortension and torque. The inner line 8 is representative of anun-deflected pocket. The outer line 9 is representative of a stressprofile when the hole is under tension, such as that shown in FIG. 9(b). The gauges located at 0 and 180 degrees sense 3× the nominal strainat this section in the drill collar. For example, if there is a nominaltensile strain, the strain at the location of these gauges is a tensilestrain 3× the nominal strain. The strain at 90 and 270 is a compressivestress equal to −⅓× the nominal strain. For example, as shown in FIG. 9(b), at W/BOB0 and W/BOB180, the strain is three times the nominalstrain. At W/BOB90 and W/BOB270, the strain is −⅓× the nominal strain.

The WOB, BOB and TOB gauges are connected into an electrical bridgearrangement. This allows the device to measure the desired measurement,while canceling out the effects of the other measurements. For example,when an axial load is applied the bridge arrangement combines the strainmeasures to calculate this load. The bridge arrangements for the BOB andthe TOB cancel out the axial load. As described in more detail withrespect to FIG. 10, the arrangement of WOB and BOB gauges, including theelectrical connection between the different types of gauge, allow formeasuring the weight on bit while canceling out the effect of bending,and for measuring the bending on bit while canceling out the effects ofstrain.

As shown in FIG. 9( d), when the portion of the drill collar 6 in thevicinity of a pocket P is subjected to pure torsion, the strain gaugesTOB₄₅ and TOB₂₂₅ are placed in compression, while strain gauges TOB₁₃₅and TOB₃₁₅ are placed in tension. The four WOB and the four BOB straingauges, however, are unaffected. FIG. 9( e) shows the effect of thestrain on the gauges. The TOB bridge arrangement, using the TOB gaugesas shown in FIGS. 9( c) and 9(d), is designed to cancel out thesestrains. Strains developed from torque loads are dissimilar and occur indifferent directions, D1 and D2, at the torque gauge locations. The TOBbridge is arranged to measure these strains.

FIG. 10 depicts a schematic diagram for measuring the weight, bending,and torque on the drill bit, showing three Wheatstone bridges, one eachfor the WOB gauges 70, the BOB gauges 90, and the TOB gauges 80.

As shown in FIG. 10, the four WOB strain gauges in the first set ofstrain gauges from each of the three pockets, P1, P2, P3, are formedinto a first Wheatstone bridge 70 comprised of twelve WOB strain gaugesarranged in four legs L₁, L₂, L₃, and L₄, with leg L₁ being opposite toleg L₃ and leg L₂ being opposite to leg L₄. For example, leg L₁ includesWOB-P1 ₀ (WOB gauge from pocket 1 at the 0° orientation), WOB-P2 ₀ (WOBgauge from pocket 2 at the 0° orientation), and WOB-P3 ₀ (WOB gauge frompocket 3 at the 0° orientation). Each leg, L₁, L₂, L₃, and L₄, usesthree strain gauges, one from a similar position in each of the pockets.As shown, the WOB strain gauges at the 0° orientation in each of thethree pockets are connected in series along leg L₁, the WOB straingauges at the 90° orientation in each of the three pockets are connectedin series along leg L₂, the WOB strain gauges at the 180° orientation ineach of the three pockets are connected in series along leg L₃, and theWOB strain gauges at the 270° orientation in each of the three pocketsare connected in series along leg L₄. The junction formed by legs L₁ andL₂ forms a first input terminal I₁, while the junction formed by legs L₃and L₄ forms a second input terminal I₂. The junction formed by legs L₂and L₃ forms a first output terminal O₁, while the junction formed bylegs L₄ and L₁ forms a second output terminal O₂.

As also shown in FIG. 10, the four TOB strain gauges in the second setof strain gauges from each of the three pockets are formed into a secondWheatstone bridge 80 comprised of twelve TOB strain gauges arranged infour legs L₁, L₂, L₃, and L₄, with leg L₁ being opposite to leg L₃ andleg L₂ being opposite to leg L₄. As shown, the TOB strain gauges at the45° orientation in each of the three pockets are connected in seriesalong leg L₁, the TOB strain gauges at the 135° orientation in each ofthe three pockets are connected in series along leg L₂, the TOB straingauges at the 225° orientation in each of the three pockets areconnected in series along leg L₃, and the TOB strain gauges at the 315°orientation in each of the three pockets are connected in series alongleg L₄. The junction formed by legs L₁ and L₂ forms a first inputterminal I₁, while the junction formed by legs L₃ and L₄ forms a secondinput terminal I₂. The junction formed by legs L₂ and L₃ forms a firstoutput terminal O₁, while the junction formed by legs L₄ and L₁ forms asecond output terminal O₂.

The BOB Wheatstone bridge 90 is similar to that described above withrespect to FIG. 6. The four BOB strain gauges in the first set of straingauges from each of the three pockets are formed into a first Wheatstonebridge 90 comprised of twelve BOB strain gauges arranged in four legsL₁, L₂, L₃, and L₄, with leg L₁ being opposite to leg L₃ and leg L₂being opposite to leg L₄. As shown, the BOB strain gauges at the 0°orientation in each of the three pockets are connected in series alongleg L₁, the BOB strain gauges at the 180° orientation in each of thethree pockets are connected in series along leg L₃, the BOB straingauges at the 90° orientation in each of the three pockets are connectedin series along leg L₂, and the BOB strain gauges at the 270°orientation in each of the three pockets are connected in series alongleg L₄.

As is conventional, in operation, voltages V are applied across the pairof input terminals I₁, I₂, I₃, of each of the bridges 70, 80, and 90.The resistance of the strain gauges in each bridge is such that when thestrain gauges are unstrained, the bridge is balanced and the voltage ΔVacross the pair of output terminals O₁, O₂, O₃, is zero. However, theresistance of the strain gauges varies proportionately with the strainso that distortion of the portion of the drill collar forming the pocketwall to which the gauges are affixed will result in a voltage drop ΔVacross the output terminals.

Importantly, as a result of the arrangement of the strain gaugesaccording to the current invention, variations in the bending load onthe drill collar 6 resulting from side forces applied to the drill bit 8will have no effect on the output voltages V of either the WOB or TOBbridges. For example, as a result of the arrangement of the straingauges according to the current invention, variations in the weightloads or torsion loads on the drill collar 6 resulting from side forcesapplied to the drill bit 8 will have no effect on the output voltages Vof the WOB, the BOB, or the TOB bridges. This is so because the neteffect of strain induced by bending is canceled out within each of thelegs of the bridges. Similarly, the weight or torque voltages cancel outsuch that the net effect of weight and torque are canceled out withineach of the legs of the BOB bridge. The differences are noted betweenthe arrangement of legs in the WOB Wheatstone bridge and the legs in theBOB Wheatstone bridge (also shown in FIG. 6). The variation in theelectrical connection cancels out the weight on bit measurements suchthat the bending on bit alone can be evaluated.

With respect to the WOB measurement, the TOB and bending have no effecton the output voltage of the WOB bridge. The general equation for aWheatstone Bridge is:ΔE=V·r(1+r)²·(ΔΔR1/R−ΔR2/R+ΔR3/R3−ΔR4/R4)Assuming a unit measurement of 1 in/in strain in the axial direction anda 0.3 in/in strain in the cross direction, then the WOB bridge factor isas follows:Ng=((1+1+1)/3−(−0.3−0.3−0.3)/3+(1+1+1)/3−(−0.3−0.3−0.3)/3)=2.6Bending is cancelled due to each arm of the bridge having a net changeof resistance of “0.” Assuming that the collar is oriented normal topocket 1 such that pocket 1 has the full bending strain, then the othertwo pockets located 120 degrees from pocket 1 have half the strain aspocket 1. The strain is also opposite that of pocket 1. Therefore, forbending:Ng=((1−0.5−0.5)/3−(−0.3+0.15+0.15)/3+(1−0.5−0.5)/3−(−0.3+0.15+0.15)/3)=0For torque, the strain in each of the WOB gauges is zero and thereforedoes not influence the measurement.

For example, a bending moment tending to bend the top of the drillcollar 6 toward the left as shown in FIG. 2 would place pocket P2 inaxial compression, as indicated in FIG. 9( a), so that, for example,gauge BOB-P2 ₀ is placed in tension, thereby increasing its resistance.However, pockets P2 and P3 would be placed in axial tension, asindicated in FIG. 9( b), so that gauges BOB-P1 ₀ and BOB-P3 ₀ are eachplaced in compression, thereby decreasing their resistance. Since thegauges BOB-P1 ₀, BOB-P2 ₀, and BOB-P3 ₀ are connected in series in legL₁ of the BOB bridge, there is no net change in the resistance of thisleg. A similar canceling out occurs in the other three legs of the BOBbridge so that the bending strain on the drill collar results in nochange in the voltage across the output terminals of the BOB bridge.Since the TOB gauges are located along lines that are oriented at 45° tothe centerline of the drill collar 6, the TOB bridge arrangement isdesigned to cancel out these strains. Strains developed from torqueloads are dissimilar and occur in different directions, D1 and D2, atthe torque gauge locations. The TOB bridge is arranged to measure thesestrains. Since the WOB gauges are connected in series in a differentmanner, the BOB bridge is also unaffected by bending strain.

With respect to the TOB measurement, the TOB uses a Wheatstone bridgethat is similar to the WOB bridge. The difference is that the TOB gaugesare oriented in such a way that they measure torque induced strains onthe collar (as described above). The gauges are mounted 45 degrees fromthe WOB gauges (Fig. X), which is the axis of the maximum principlestrains for torque in the collar. Torque develops tensile strain on twogauges in each pocket that re opposite to each other, and compressivestress in the other two gauges. The bridge circuit is arranged such thatthe similar stress gauges are in opposite legs of the bridge. The bridgefactor then becomesNg=(1+1+1)/3−(−1−1−1)/3+(1+1+1)/3−(−1−1−1)/3=4The WOB strains at the TOB gauges are identical for all gauges.Therefore, the effect of WOB on the TOB bridge is:Ng=(1+1+1)/3−(1+1+1)/3+(1+1+1)/3−(1+1+1)/3=0Therefore, WOB strains are self-canceling for the TOB bridge.

Similarly, consider the bending moment tending to bend the top of thedrill collar 6, as described above with respect to the BOB bridge, thatplaces pocket P2 in axial compression, as indicated in FIG. 7( a), sothat, for example, gauge WOB-P2 ₀ is placed in tension, therebyincreasing its resistance. However, pockets P2 and P3 would be placed inaxial tension, as indicated in FIG. 7( b), so that gauges WOB-P1 ₀ andWOB-P3 ₀ are each placed in compression, thereby decreasing theirresistance. Since the gauges WOB-P1 ₀, WOB-P2 ₀, and WOB-P3 ₀ areconnected in series in leg L₁ of the WOB bridge, there is no net changein the resistance of this leg. A similar canceling out occurs in theother three legs of the WOB bridge so that the bending strain on thedrill collar results in no change in the voltage across the outputterminals of the WOB bridge. Since the TOB gauges are located alonglines that are oriented at 45° to the centerline of the drill collar 6,the TOB bridge is also unaffected by bending strain.

As described above, with respect to the BOB measurement, the WOB and TOBhave no effect on the output voltage of the BOB bridge.

The strain indicated by the WOB, TOB, and BOB bridges 70, 80, and 90 canbe determined from the voltage ΔV across their output terminals by theequations:ε_(WOB) =[ΔV/V]·[4/4.2K _(g)]ε_(TOB) =[ΔV/V]·[4/12K _(g)]ε_(BOB) =[ΔV/V]·[4/4.2K _(g)]where:

ε_(WOB)=the strain indicated by the WOB bridge 70

ε_(TOB)=the strain indicated by the TOB bridge 80

ε_(BOB)=the strain indicated by the BOB bridge 90

V=the voltage applied across the input terminals of the bridge

ÿV=the voltage drop across the output terminals of the bridge

K_(g)=the gauge factor for the strain gauge (from the gaugemanufacturer)

The weight, torque, and bending on the drill bit are determined fromthese strains by the equations:WOB=[ε_(WOB) ·E·A]/k _(t)TOB=[ε_(TOB) ·J·G]/[R·k _(t)]BOB=[ε_(TOB) ·E·I]/[R·k _(t)]where:

WOB=the weight on the drill bit

TOB=the torque on the drill bit

BOB=the bending on the drill bit

E=the modulus of elasticity for the drill collar material

G=the shear modulus for the drill collar material

A=the cross-sectional area of the drill collar

J=the torsional modulus for the drill collar

R=the radius of the drill collar

k_(t)=the stress concentration factor for the pocket

As shown in FIG. 10, the voltage drops ΔV from the WOB, TOB, and BOBbridges 70, 80, and 90 and are amplified by amplifiers 40, 42, and 43,respectively, and then sensed by conventional voltage measuring devicesincorporated into the circuit board 30. The output signals S₁, S₂ and S₃from the voltage measuring devices, which are representative of thestrain sensed by the WOB, TOB, and BOB gauges, respectively, are sent toa microprocessor 32, where they are digitized. Using these digitizedvalues, the microprocessor 32 is programmed to perform the computationsdiscussed above so as to arrive at the weight and torque on the drillbit. This information is sent to a mud pulse telemetry system 50 fortransmission to the surface using the mud pulser 5, where it is detectedby the data acquisition system 12.

Preferably, annulus and bore pressure transducers as well as atemperature sensor are incorporated into the drill collar 6 to permittemperature and pressure compensation. Using techniques well known inthe art, the microprocessor uses the pressure measurement to calculatethe strain due to pressure and then subtract or add this from theapparent strain to get the true WOB and TOB strains. Similarly, based ona curve supplied by the gauge manufacture, which is also programmed intothe microprocessor, temperature correction is also performed for thestrain gauges.

Also, although in the preferred embodiment, four strain gauges for eachWheatstone bridge are used, the invention could also be practiced usedonly two TOB strain gauges provided that they oppose each other—forexample, TOB-P2 ₄₅ and TOB-P2 ₂₂₅ or TOB-P2 ₁₃₅ and TOB-P2 ₃₁₅. In thiscase, precision resistors would be used in the other two legs to balancethe bridge.

Although in the embodiment discussed above, three pockets P areutilized, any greater number of pockets could also be utilized providedthat the pockets are circumferentially spaced equidistantly and thestrain gauges in each of the pockets are oriented as discussed above andprovided that each of the gauges oriented in the same location in eachpocket (e.g., each of the 0° gauges) are connected into the same leg ofthe bridge. Moreover, although in the embodiment discussed above, all ofthe gauges within each pocket are located in a common plane orientedperpendicularly to the axis of the pocket, the gauges could be locatedalong different planes oriented perpendicularly to the axis of thepocket but displaced from each other along that axis, provided that eachpair of opposing gauges (e.g., the 0° and 180° pair of gauges) arelocated in approximately the same plane. Moreover, although in theembodiment discussed above both the WOB and TOB are located in the samepocket, the WOB gauges could be located in one set of at least threeequidistantly spaced pockets and the TOB gauges located in another,independent set of at least three equidistantly spaced pockets. Althoughin the embodiment discussed above, the pockets are formed into thesection of drill pipe forming the drill collar, other sections of thedrill string could also be utilized.

FIG. 11 shows an alternate embodiment in which the BOB and TOB straingauges 39 are circumferentially spaced around the bottom wall 35 of eachpocket P, and the WOB gauges arranged on the side wall. Alternatively,the BOB gauges could be arranged on the side wall 38 and the WOB and TOBgauges arranged on the bottom wall 35, or the TOB gauges could bearranged on the side wall with either of the BOB or WOB gauges arrangedon the bottom wall.

Although in the embodiment discussed above, three pockets P areutilized, any greater number of pockets could also be utilized providedthat the pockets are circumferentially spaced equidistantly and thestrain gauges in each of the pockets are oriented as discussed above andprovided that each of the gauges oriented in the same location in eachpocket (e.g., each of the 0° gauges) are connected into the same leg ofthe bridge. Moreover, although in the embodiment discussed above, all ofthe gauges within each pocket are located in a common plane orientedperpendicularly to the axis of the pocket, the gauges could be locatedalong different planes oriented perpendicularly to the axis of thepocket but displaced from each other along that axis, provided that eachpair of opposing gauges (e.g., the 0° and 180° pair of gauges) arelocated in approximately the same plane. Moreover, although in theembodiment discussed above both the WOB and TOB are located in the samepocket, the WOB gauges could be located in one set of at least threeequidistantly spaced pockets and the TOB gauges located in another,independent set of at least three equidistantly spaced pockets. Althoughin the embodiment discussed above, the pockets are formed into thesection of drill pipe forming the drill collar, other sections of thedrill string could also be utilized.

As shown in FIG. 12, a passage 82 formed in the drill collar 6 allowsthe pocket 37 to be purged with an inert gas, such as helium ornitrogen, to prevent degradation of the strain gauges 39. The cap 36 canbe secured within the pocket 37 via a snap ring 81, and the pocketsealed from the drilling mud 16 by an O-ring (not shown), as discussedin the aforementioned U.S. Pat. No. 6,547,016. Alternatively, a metallicseal, such as a C-shaped metallic seal, may be used to prevent drillingmud 16 from entering the pocket. Unlike elastomeric seals, such asO-rings, metallic seals do not degrade with time and temperature, arenot permeable, and do not suffer from explosive decompression whenrepeatedly exposed to high and low pressures. Suitable metallic sealscan be made from Inconel 718, A286, NP35N, 17-7PH or other metals havinggood corrosion resistance. In the embodiment shown in FIG. 12, ametallic gland seal 81 is used to form a seal between the pocket 37 andthe pocket cover 36. In another embodiment, a metallic face seal 82 isused, as shown in FIG. 13. In this embodiment, a threaded cap 36′ isused to increase the pre-load applied to the seal 82. In another exampleembodiment, the inert gas filling port may be incorporated into thecover itself. A seal, such as a metallic seal or an elastomeric o-ringmay function as a first seal to prevent drilling fluids from enteringthe pocket. This seal, acting as a first seal, may also further keep themetallic seal clean and not exposed to the drilling fluid at least for aperiod of time.

Accordingly, it should be realized that the present invention may beembodied in other specific forms without departing from the spirit oressential attributes thereof and that reference should be made to theappended claims, rather than to the foregoing specification, asindicating the scope of the invention.

What is claimed:
 1. An apparatus for sensing the bending applied to adrill bit coupled to a drill string operating down hole in a well,comprising: a) a drill pipe, said drill pipe defining a centerlinethereof and having means for being coupled into a drill string; b) atleast first, second and third pockets formed in said drill pipe, saidpockets circumferentially spaced approximately equidistantly around saiddrill pipe, each of said pockets forming at least one wall; c) a set ofstrain sensors for each of said pockets, each of said sets of strainsensors affixed to said wall of its respective pocket, each of said setsof strain sensors comprising first, second, third and fourth strainsensors circumferentially spaced approximately equidistantly around saidwall of its respective pocket, each of said first strain sensors in eachof said sets of strain sensors disposed opposite said third strainsensor in its respective set, each of said second strain sensors in eachof said sets of strain sensors disposed opposite said fourth strainsensor in its respective set, said first and third strain sensorsdisposed along a line parallel to said centerline of said drill pipe,said second and fourth strain sensors disposed along a lineperpendicular to said centerline of said drill pipe; e) circuitryconnecting each of said strain sensors in said sets, said circuitryforming a bridge, said bridge comprising first, second, third and fourthlegs, (i) said first leg of said bridge being opposite to said third legof said bridge, (ii) said second leg of said bridge being opposite saidfourth leg of said bridge, (iii) each of said first strain sensors ineach of said sets of strain sensors connected in series along said firstleg of said bridge, (iv) each of said second strain sensors in each ofsaid sets of strain sensors connected in series along said second leg ofsaid bridge, (v) each of said third strain sensors in each of said setsof strain sensors connected in series along said third leg of saidbridge, and (vi) each of said fourth strain sensors in each of said setsof strain sensors connected in series along said fourth leg of saidbridge.
 2. The apparatus according to claim 1, further comprising a capfor closing at least one of the pockets, a metallic seal cooperatingwith the cap to seal the pocket from fluid surrounding the drill string.3. The apparatus according to claim 1, wherein each of said sets ofstrain sensors are affixed to a side wall of its respective pocket. 4.The apparatus according to claim 1, wherein each of said sets of strainsensors are affixed to a bottom wall of its respective pocket.
 5. Theapparatus according to claim 1, wherein (i) a first junction is formedbetween said first and second legs, (ii) a second junction is formedbetween said third and fourth legs, whereby said first and secondjunctions form a first pair of terminals, (iii) a third junction isformed between said first and fourth legs, (iv) a fourth junction isformed between said second and third legs, whereby said third and fourthjunctions form a second pair of terminals.
 6. The apparatus according toclaim 5, further comprising: f) means for applying a voltage across oneof said pairs of terminals; g) means for sensing a voltage across theother of said pair of terminals; h) means for determining said bendingon said drill bit from said sensed voltage.
 7. An apparatus for sensingbending, weight, and torque applied to a drill bit operating down holein a well, comprising: a) a drill bit; b) a drill string operativelycoupled to said drill bit, said drill string having a section disposedproximate said drill bit, said section of said drill string defining acenterline thereof; c) at least first, second and third pockets formedin said section of said drill string, said pockets circumferentiallyspaced approximately equidistantly around said section of said drillbit, each of said pockets forming at least first and second walls; d) afirst set of strain sensors for each of said pockets, each of said firstsets of strain sensors affixed to one of said walls of its respectivepocket, each of said first sets of strain sensors comprising first,second, third and fourth strain sensors circumferentially spacedapproximately equidistantly around said one of said walls of itsrespective pocket, each of said first strain sensors in each of saidfirst sets of strain sensors disposed opposite said third strain sensorin its respective set, each of said second strain sensors in each ofsaid first sets of strain sensors disposed opposite said fourth strainsensor in its respective set, each of first and third strain sensors ineach of said first sets of strain sensors disposed along a first lineapproximately parallel to said centerline of said section of said drillstring, each of said second and fourth strain sensors in each of saidfirst sets of strain sensors disposed along a second line approximatelyperpendicular to said centerline of said section of said drill string;e) first circuitry connecting each of said strain sensors in said firstsets of strain sensors, said first circuitry forming a first bridge,said first bridge comprising first, second, third and fourth legs, (i) afirst junction formed between said first and second legs, (ii) a secondjunction formed between said third and fourth legs, whereby said firstand second junctions form a first pair of terminals, (iii) a thirdjunction formed between said first and fourth legs, (iv) a fourthjunction formed between said second and third legs, whereby said thirdand fourth junctions form a second pair of terminal, (v) each of saidfirst strain sensors in each of said first sets of strain sensorsconnected in series along said first leg of said first bridge, (vi) eachof said second strain sensors in each of said first sets of strainsensors connected in series along said second leg of said first bridge,(vii) each of said third strain sensors in each of said first sets ofstrain sensors connected in series along said third leg of said firstbridge, and (viii) each of said fourth strain sensors in each of saidfirst sets of strain sensors connected in series along said fourth legof said first bridge; f) means for applying a voltage across said one ofsaid first and second pairs of terminals of said first bridge; g) meansfor sensing a voltage across the other of said first and secondterminals of said first bridge; h) means for determining said weight onsaid drill bit from said voltage sensed across said first bridge; i) asecond set of strain sensors for each of said pockets, each of saidsecond sets of strain sensors affixed to one of said walls of itsrespective pocket, each of said second sets of strain sensors comprisingat least fifth and sixth strain sensors spaced around said one of saidwalls of its respective pocket, each of said fifth strain sensors ineach of said second sets of strain sensors disposed opposite said sixthstrain sensor in its respective set, each of fifth and sixth strainsensors in each of said second sets of strain sensors disposed along athird line oriented approximately 45° to said first line; j) secondcircuitry connecting each of said strain sensors in said second sets ofstrain sensors, said circuitry forming a second bridge, said secondbridge comprising first, second, third and fourth legs, said first legbeing opposite said third leg, each of said fifth strain sensors in eachof said second sets of strain sensors connected in series along saidfirst leg of said second bridge, each of said sixth strain sensors ineach of said sets of strain sensors connected in series along said thirdleg of said second bridge, said second bridge having a pair of inputterminals and a pair of output terminals; k) means for applying avoltage across said input terminals of said second bridge; l) means forsensing a voltage across said output terminals of said second bridge; m)means for determining said torque on said drill bit from said voltagesensed across said output terminals of said second bridge n) a third setof strain sensors for each of said pockets, each of said third sets ofstrain sensors affixed to one of said walls of its respective pocket,each of said third sets of strain sensors comprising ninth, tenth,eleventh and twelfth strain sensors circumferentially spacedapproximately equidistantly around said one of said walls of itsrespective pocket and positioned radially with respect to each of saidfirst sets of strain sensors affixed to one of said walls of itsrespective pocket, each of said ninth strain sensors in each of saidthird sets of strain sensors disposed opposite said tenth strain sensorin its respective set, each of said eleventh strain sensors in each ofsaid third sets of strain sensors disposed opposite said twelfth strainsensor in its respective set, each of ninth and tenth strain sensors ineach of said third sets of strain sensors disposed along a first lineapproximately parallel to said centerline of said section of said drillstring, each of said eleventh and twelfth strain sensors in each of saidthird sets of strain sensors disposed along a second line approximatelyperpendicular to said centerline of said section of said drill string;o) third circuitry connecting each of said strain sensors in said thirdsets of strain sensors, said third circuitry forming a third bridge,said third bridge comprising first, second, third and fourth legs, (i) afirst junction formed between said first and second legs in said thirdcircuitry, (ii) a second junction formed between said third and fourthlegs in said third circuitry, whereby said first and second junctionsform a first pair of terminals, (iii) a third junction formed betweensaid first and fourth legs in said third circuitry, (iv) a fourthjunction formed between said second and third legs in said thirdcircuitry, whereby said third and fourth junctions form a second pair ofterminal, (v) each of said ninth strain sensors in each of said thirdsets of strain sensors connected in series along said first leg of saidfirst bridge, (vi) each of said eleventh strain sensors in each of saidthird sets of strain sensors connected in series along said second legof said third bridge, (vii) each of said tenth strain sensors in each ofsaid third sets of strain sensors connected in series along said thirdleg of said third bridge, and (viii) each of said twelfth strain sensorsin each of said third sets of strain sensors connected in series alongsaid fourth leg of said third bridge; p) means for applying a voltageacross said input terminals of said second bridge; q) means for sensinga voltage across said output terminals of said second bridge; r) meansfor determining said bending on said drill bit from said voltage sensedacross said output terminals of said second bridge.
 8. The apparatusaccording to claim 7, wherein said second wall of each of said pocketsis a bottom wall, wherein each of said second sets of strain sensors areaffixed to said bottom wall of its respective pocket.
 9. The apparatusaccording to claim 7, further comprising a cap for closing at least oneof the pockets, a metallic seal cooperating with the cap to seal thepocket from fluid surrounding the drill string.
 10. The apparatusaccording to claim 7, wherein a limit on the variation of readings fromeach of the strain sensors in said first sets of strain sensors isindicative of a rotation of the drill string.
 11. The apparatusaccording to claim 7, wherein if the drill string is rotating, thebending moment is equal to one-half of the difference between themaximum and minimum weight determined over a time longer than a rotationperiod.
 12. The apparatus according to claim 7, wherein said first wallof each of said pockets is a side wall, and wherein each of said firstsets of strain sensors are affixed to said side wall of its respectivepocket.
 13. The apparatus according to claim 12, wherein each of saidsecond sets of strain sensors are affixed to said side wall of itsrespective pocket.
 14. The apparatus according to claim 7, wherein saidfirst wall of each of said pockets is a bottom wall, wherein each ofsaid first sets of strain sensors are affixed to a bottom wall of itsrespective pocket.
 15. The apparatus according to claim 14, wherein eachof said second sets of strain sensors are affixed to said bottom wall ofits respective pocket.
 16. The apparatus according to claim 14, whereineach of said second sets of strain sensors are affixed to a side wall ofits respective pocket.